1. Field of the Invention
The present invention relates to a drying apparatus for removing water content from liquid, semisolid or solid substance to be dried. Such drying apparatus comprises a drying vessel whose inner wall provides a heat conduction surface for transmitting heat to wet substance, and circulating rotary means to put the substance in motion in the vessel, thereby increasing the efficiency with which the substance can be brought to the heat conduction surface.
2. Description of the Prior Art
A drying apparatus is used for removing water content from liquid, semisolid or solid substance, such as cornstarch, beancurd refuse or water-and-powder mixture. Substances to be dried range from liquid or slurry substance or semi-solid to wet solid or powder-like substance. Such drying apparatus has a drying vessel to be loaded with substance to be dried. The drying vessel has heating means and a heat conduction surface for transmitting heat to the substance to be dried. Usually, such heating means uses steam which is supplied from a boiler. In an attempt to increase the amount of water vaporization per unit time the efficiency with which the substance is brought in contact with the heat conduction surface of the drying vessel is increased by putting the substance in motion relative to the heat conduction surface of the vessel. A conventional drying apparatus uses a circulator which has rotary paddles fixed to a rotary axle extending horizontally with respect to the direction of gravity. Its drying vessel has a jacket to be supplied with steam for heating the inner wall of the vessel, thereby permitting it to function as a heat conduction surface. In operation the paddles are rotated to bring the substance into contact with the inner wall of the vessel, which is heated and functions as heat conduction surface.
Another conventional drying apparatus uses a circulator which has disks fixed to a rotary axle extending horizontally with respect to the direction of gravity. Its drying vessel has a jacket to be supplied with steam for heating the inner wall of the vessel, and each disk has an inner hollow space to be supplied with steam for heating its surface, thereby permitting the disk surface to function as a heat conduction surface, too. In operation the substance is brought into contact with the heated inner wall of the vessel and the heated surface of each disk, both functioning as heat conduction surface.
These drying apparatuses are satisfactory in operation, but they have still disadvantages as follows. As for the drying apparatus using rotary paddles the paddles must be rotated against the resistance which is caused by the substance in the vessel, and therefore the paddles cannot be rotated at an increased speed. There is a fear of breaking the paddles if the rotation of paddles per unit time is increased in the hope of increasing the dewatering efficiency of the drying apparatus. A certain substance is liable to attach to the paddles when they rotate at an increased speed. Then, the substance is rotated along with the rotating paddles, and therefore it cannot be put in flowing motion on the heat conduction surface. This tendency increases with increased viscosity. For these reasons the paddles cannot be rotated at such an increased speed that the substance may be brought in contact with the heat conduction surface at an increased efficiency.
As for the drying apparatus using heating disks the dewatering efficiency can be increased by increasing the number of the heating disks. This will increase the total weight of the disks to be supported by a rotary axle, which extends horizontally relative to the direction of gravity. The axle must be stout, and accordingly its weight increases. As a consequence it is difficult to increase the rotation per unit time of the rotary axle and associated heating disks. This will limit the drying apparatus to a reduced dewatering efficiency. If the rotating speed of the heating disks is increased, the power of the driving motor must be increased. Disadvantageously the use of an increased power drive will cost much.
Summary of the Invention: One object of the present invention is to provide a drying apparatus which is capable of circulating wet substance at an increased speed and bringing the substance into contact with the heat conduction surface of the drying apparatus at an increased efficiency, thereby improving the dewatering efficiency of the drying apparatus.
To attain this object a drying apparatus for removing water content from liquid, semisolid or solid substance to be dried comprising: a drying vessel to contain said substance, said vessel having a heat conduction surface on its inner wall for transmitting heat to said substance: and circulating rotary means to put said substance in motion in said vessel, thereby increasing the efficiency with which said substance can be brought to said heat conduction surface, is improved according to the present invention in that said circulating rotary means comprises a rotary shaft vertically extending in said vessel in the direction of gravity, and a spiral blade integrally connected to and wound around said rotary shaft, said spiral blade having a flat upper surface, whereby rotation of said rotary shaft and hence said spiral blade may cause said substance to rise up in the direction of gravity, and slide on said flat upper surface of said spiral blade until the so raised substance is allowed to fall down in the direction of gravity through a falling space which is defined in said drying vessel, and until said substance has come in contact with said heat conduction surface.
Said drying vessel may be a hollow cylinder which is coaxial with said rotary shaft; the inner surface of said hollow cylinder may provide said heat conduction surface; and said spiral blade may be located close to said inner surface of said hollow cylinder but leaving a gap large enough to allow said spiral blade to rotate, thereby permitting said spiral blade to rotate and raise said substance while keeping said substance in contact with said heat conduction surface.
In operation wet substance is put in the drying vessel, and then the substance is located at a lower position under the influence of gravity. The heat conduction surface of the drying vessel is heated by heating means so that heat is transfered to the wet substance in the drying vessel. The spiral blade is made to rotate by rotating its axle. In case where the spiral blade is hollow and where heating medium is led into the inside of the spiral blade, the upper surface of the spiral blade will function as heat conduction surface like the inner wall of the drying vessel.
The wet substance layed on the upper surface of the spiral blade will slide thereon while the spiral blade rotates. Thus, the wet substance will be renewed to come to contact with the heat conduction surface of the drying vessel at an increased efficiency. The rotation of the spiral blade will cause the substance on the blade surface to rise up in the direction of gravity. When the substance is raised at a higher level, it will fall down in the direction of gravity through the falling space in the drying vessel. Thus, the substance rises and falls repeatedly in the drying vessel. The substance on the rotating spiral blade will slide along the blade surface because of inertia. The increase of the blade rotation will increase the slide speed of the substance on the blade surface.
The rotation of the spiral blade will raise the substance. Therefore, the increase of the rotation per unit time of the spiral blade will expedite the circulating motion of the substance in the drying vessel, bringing the substance into contact with the heat conduction surface both of the upper surface of the blade and the inner wall of the drying vessel. Accordingly the dewatering efficiency will be increased.
Specifically, when the spiral blade is rotated, the substance will be rotated because of friction between the substance and the blade surface, and will be raised. The centrifugal force will be applied to the substance on the spiral blade, thereby pushing the substance outwards until it has come to contact with the inner wall of the drying vessel. Thus, the substance will be made to rise and fall while being brought into contact with the heat conduction surface both of the spiral blade the inner wall of the drying vessel all the time.